The present invention relates to electrical isolators and in particular to electrical isolators that employ microelectromechanical system (MEMS) devices.
Electrical isolators are used to provide electrical isolation between circuit elements for the purposes of voltage level shifting, electrical noise reduction, and high voltage and current protection. Circuit elements can be considered electrically isolated if there is no path in which a direct current (DC) can flow between them. Isolation of this kind can be obtained by capacitive or inductive coupling. In capacitive coupling, an electrical input signal is applied to one plate of a capacitor to transmit an electrostatic signal across an insulating dielectric to a second plate at which an output signal is developed. In inductive coupling, an electrical input signal is applied to a first coil to transmit an electromagnetic field across an insulating gap to a second coil, which generates the isolated output signal.
Both such isolators essentially block steady state or DC electrical signals. Such isolators, although simple, block the communication of signals that have significant low frequency components. Further, these isolators can introduce significant frequency dependent attenuation and phase distortion in the transmitted signal. These features make such isolators unsuitable for many types of signals including many types of high-speed digital communications.
In addition, it is sometimes desirable to provide high voltage ( greater than 2 kV) isolation between two different portions of a system, while maintaining a communication path between these two portions. This is often true in industrial control applications where it is desirable to isolate the sensor/actuator portions from the control portions of the overall system. It is also applicable to medical instrumentation systems, where it is desirable to isolate the patient from the voltages and currents within the instrumentation.
The isolation of digital signals is frequently provided by optical isolators. In an optical isolator, an input signal drives a light source, typically a light emitting diode (LED) positioned to transmit its light to a photodiode or phototransistor through an insulating but transparent separator. Such a system will readily transmit a binary signal of arbitrary frequency without the distortion and attenuation introduced by capacitors and inductors. The optical isolator further provides an inherent signal limiting in the output through saturation of the light receiver, and signal thresholding in the input, by virtue of the intrinsic LED forward bias voltage.
Nevertheless, optical isolators have some disadvantages. They require a relatively expensive gallium arsenide (GaAs) substrate that is incompatible with other types of integrated circuitry and thus optical isolators often require separate packaging and assembly from the circuits they are protecting. The characteristics of the LED and photodetector can be difficult to control during fabrication, increasing the costs if unit-to-unit variation cannot be tolerated. The power requirements of the LED may require signal conditioning of the input signal before an optical isolator can be used, imposing yet an additional cost. While the forward bias voltage of the LED provides an inherent noise thresholding, the threshold generally cannot be adjusted but is fixed by chemical properties of the LED materials. Accordingly, if different thresholds are required, additional signal conditioning may be needed. Additionally, the LED is a diode and thus limits the input signal to a single polarity unless multiple LEDs are used.
Further, optical isolators are not well suited for the isolation of analog signals. Optical isolators can only operate to isolate analog signals in one of two ways. One of these is to operate the LED of the optical isolator in its linear range, so that the output signal of the optical isolator accurately reflects the input signal. Maintaining the operation of the LED in its linear range is difficult to do consistently (as is the calibration required to determine what is the LED""s linear range). The second way of isolating analog signals is to digitize the analog signal and transmit the digitized bits with multiple optical isolators. Multiple isolators, however, are expensive and bulky and the need to preprocess the input analog signal requires a significant amount of electronics.
Other technologies also exist or are being developed that can be employed to isolate digital and analog signals. For example, U.S. patent application Ser. No. 09/788,928 filed on Feb. 20, 2001, which is hereby incorporated by reference, discloses a mechanical isolator that is manufactured using MEMS techniques and suitable for transmitting digital signals. Similarly, U.S. patent application Ser. No. 09/804,817 filed on Mar. 13, 2001, also hereby incorporated by reference, discloses a MEMS isolator suitable for transmitting analog signals. In each case, the isolator uses a specially fabricated microscopic beam supported on a substrate and whose ends are insulated from each other. One end of the beam is connected to a microscopic actuator, which receives an input signal to move the beam against a biasing force provided by a biasing device. The other end of the beam is attached to a sensor detecting movement of the beam. For the digital isolator, the biasing force is constant, and beam movement occurs only when the input signal is sufficient to overcome the biasing force.
Although such MEMS devices can provide signal isolation, the devices by themselves cannot be implemented as isolated analog-to-digital converters (isolated-ADCs). To the extent such MEMS devices are employed as isolated analog-to-analog converters, the output of the devices can be converted into digital format by the addition of a conventional analog-to-digital conversion circuit, thus producing isolated-ADCs. However, the addition of this circuit adds to the expense of the MEMS devices. Further, while it is possible to design converters that operate open-loop, closed-loop converters are preferable in order to maintain desired linear operation of the converters over a relatively large range of possible input signals. Consequently, designing isolated-ADCs using conventional MEMS devices that are employed as isolated analog-to-analog converters not only requires that conventional analog-to-digital conversion circuits be provided at the output of the MEMS devices, but also requires feedback circuitry such as a proportional-integral control circuit. Because the feedback circuitry, conventional analog-to-digital conversion circuitry and MEMS devices are typically physically located on different microchips, the costs associated with designing and constructing isolated-ADCs by way of these conventional devices is further increased.
Therefore, it would be desirable if a new isolated-ADC that employed a MEMS device was developed, where the new isolated-ADC employed simpler, less costly and more easily manufactured circuitry.
The present invention provides a microelectromechanical system (MEMS) circuit in which the MEMS device forms part of a sigma-delta converter. The converter provides stability, a simplified design, and a digitized output such that the circuit acts as an isolated analog-to-digital converter (isolated-ADC) with a digital output that can be used for later computerized processing.
Generally, an analog signal that is input to the MEMS device is converted into a force applied to a beam within the MEMS device. Also applied to the beam is a feedback signal. The combined forces upon the beam move the beam with respect to a sensor, which outputs a signal indicative of the position of the beam. The signal is compared with a reference value representative of a reference position of the beam, and the result of the comparison is provided as a digital signal that is used to generate the output signal as well as the feedback signal.
In particular, the present invention relates to an isolated-ADC providing isolation between an analog input signal and a digital output signal. The isolated-ADC includes a microelectromechanical system (MEMS), a comparator, and a digital-to-analog converter (DAC). The MEMS includes a substrate, a beam element supported from the substrate for movement with respect to an axis relative to the substrate, and a first actuator attached to the beam element, where the first actuator is capable of exerting a first force upon the beam element causing the beam element to move with respect to the axis, and where the first force is dependent upon the analog input signal provided to the isolated-ADC. The MEMS further includes a sensor communicating with the beam element to detect a change in position of the beam element and to produce a position signal indicative of the position of the beam element, and a second actuator attached to the beam element, where the second actuator is capable of exerting a second force upon the beam element based upon a feedback signal. The comparator is electrically coupled to the sensor, and generates a digital signal based upon a comparison of the position signal with a reference value representative of a reference position of the beam element. The DAC is electrically coupled between the second actuator and the comparator, and generates the feedback signal at least in partial dependence upon the digital signal. The digital output signal is further produced by a processing device within the isolated-ADC in dependence upon the digital signal, the digital output signal being an indication of, and electrically isolated from, the analog input signal.
The present invention further relates to an isolated-ADC providing isolation between an analog input signal and a digital output signal. The isolated-ADC includes a microelectromechanical system (MEMS), a comparator, and a differentiator. The MEMS includes a substrate, a beam element supported from the substrate for movement with respect to an axis relative to the substrate, and a first actuator attached to the beam element. The first actuator is capable of exerting a first force upon the beam element causing the beam element to move with respect to the axis, and the first force is dependent upon the analog input signal provided to the isolated-ADC. The MEMS further includes a sensor communicating with the beam element to detect a change in position of the beam element and to produce a position signal indicative of the position of the beam element, and a second actuator attached to the beam element, where the second actuator is capable of exerting a second force upon the beam element based upon a feedback signal. The comparator is electrically coupled to the sensor, and generates a digital signal based upon a comparison of the position signal with a reference value representative of a reference position of the beam element. The differentiator is electrically coupled to the comparator, and generates an intermediate signal related to a derivative of the digital signal. The first feedback signal includes at least one of a first analog signal component based upon the intermediate signal and a second analog signal component based upon the digital signal. The digital output signal is further produced by a processing device within the isolated-ADC in dependence upon the digital signal, the digital output signal being an indication of, and electrically isolated from, the analog input signal.
The present invention further relates to an isolated-ADC providing isolation between an analog input signal and a digital output signal. The isolated-ADC includes a microelectromechanical system (MEMS), a set of comparators, a logical decision device, and a digital-to-analog converter (DAC). The MEMS includes a substrate, a beam element supported from the substrate for movement with respect to an axis relative to the substrate, and a first actuator attached to the beam element, where the first actuator is capable of exerting a first force upon the beam element causing the beam element to move with respect to the axis, and where the first force is dependent upon the analog input signal provided to the isolated-ADC. The MEMS further includes a sensor communicating with the beam element to detect a change in position of the beam element and to produce a position signal indicative of the position of the beam element, and a second actuator attached to the beam element, where the second actuator is capable of exerting a second force upon the beam element based upon a feedback signal. The set of comparators includes at least first, second and third comparators that are each electrically coupled to the sensor, where the first comparator generates a first digital signal based upon a comparison of the position signal with a reference value representative of a reference position of the beam element, the second comparator generates a second digital signal based upon a comparison of the position signal with a first offset value representative of a first reference position offset of the beam element, and the third comparator generates a third digital signal based upon a comparison of the position signal with a second offset value representative of a second reference position offset of the beam element. The logical decision device is coupled to the first, second and third comparators, and generates a feedback bitstream signal that is based on at least the first, second and third digital signals. The DAC is electrically coupled between the second actuator and the logical decision device, and generates the feedback signal in dependence upon the feedback bitstream signal. The digital output signal is further produced by the isolated-ADC in dependence upon at least one of the first, second and third digital signals, the digital output signal being an indication of, and electrically isolated from, the analog input signal.
The present invention additionally relates to an isolated-ADC providing isolation between an analog input signal and a digital output signal. The isolated-ADC includes a microelectromechanical system (MEMS), a differential amplifier, a comparator bias circuit, and a digital-to-analog converter (DAC). The MEMS includes a substrate, a beam element supported from the substrate for movement with respect to an axis relative to the substrate, and a first actuator attached to the beam element, where the first actuator is capable of exerting a first force upon the beam element causing the beam element to move with respect to the axis, and where the first force is dependent upon the analog input signal provided to the isolated-ADC. The MEMS further includes a sensor communicating with the beam element to detect a change in position of the beam element and to produce a position signal indicative of the position of the beam element, and a second actuator attached to the beam element, where the second actuator is capable of exerting a second force upon the beam element based upon a feedback signal. The differential amplifier is electrically coupled to the sensor, and generates two intermediate signals based upon a comparison of the position signal with a reference value representative of a reference position of the beam element. The comparator bias circuit includes a comparator that receives the two intermediate signals and in response generates a digital signal. The DAC is electrically coupled between the second actuator and the comparator bias circuit, and generates the feedback signal dependent upon the digital signal. The digital output signal is further produced by the isolated-ADC in dependence upon the digital signal, the digital output signal being an indication of, and electrically isolated from, the analog input signal.
The present invention further relates to an isolated-ADC providing isolation between an analog input signal and a digital output signal. The isolated-ADC includes a microelectromechanical system (MEMS), a first comparator and a digital-to-analog converter (DAC). The MEMS includes a substrate, a beam element supported from the substrate for movement with respect to an axis relative to the substrate, and a first actuator attached to the beam element, where the first actuator is capable of exerting a first force upon the beam element causing the beam element to move with respect to the axis, and where the first force is dependent upon the analog input signal provided to the isolated-ADC. The MEMS further includes a sensor communicating with the beam element to detect a change in position of the beam element and to produce a position signal indicative of the position of the beam element, a second actuator attached to the beam element, where the second actuator is capable of exerting a second force upon the beam element based upon a feedback signal, and a damping element coupled to the beam element, where the damping element tends to generate a third force when the beam element moves, the third force tending to counter the at least one of the first and second forces causing the movement. The first comparator is electrically coupled to the sensor, and generates a first digital signal based upon a comparison of the position signal with a reference value representative of a reference position of the beam element. The DAC is electrically coupled between the first comparator and the second actuator, and generates the feedback signal in dependence upon the first digital signal. The digital output signal is further produced by the isolated-ADC in dependence upon the first digital signal, the digital output signal being an indication of, and electrically isolated from, the analog input signal.
The present invention further relates to an isolated-ADC. The isolated-ADC includes a microelectromechanical means for adding an analog input signal to a feedback signal and producing a position signal in response to the analog input and feedback signals, and a means for generating a digital output signal based upon the position signal. The isolated-ADC further includes a means for generating the feedback signal based upon the position signal, where the digital output signal is electrically isolated from the analog input signal.
The present invention additionally relates to a method of providing a digital output signal based upon an analog input signal, where the digital output signal is electrically isolated from the analog input signal. The method includes receiving the analog input signal at a first actuator of a microelectromechanical system (MEMS), receiving a feedback signal at a second actuator of the MEMS, and generating movement of a beam element of the MEMS by way of the first and second actuators in response to the respective analog input and feedback signals. The method further includes sensing a position of the beam element of the MEMS at a sensor of the MEMS, comparing the sensed position with a reference value, generating a first digital signal in response to the comparing of the position and the reference value, where the first digital signal is at a high level while the sensed position is determined to be greater than the reference value and at a low level while the sensed position is determined to be less than the reference value, and generating, based upon the first digital signal, both the digital output signal and the feedback signal.
The present invention further relates to a method of providing a digital output signal based upon an analog input signal, where the digital output signal is electrically isolated from the analog input signal. The method includes receiving the analog input signal at a first actuator of a microelectromechanical system (MEMS), receiving a first feedback signal at a second actuator of the MEMS, and generating movement of a beam element of the MEMS by way of the first and second actuators in response to the respective analog input and first feedback signals. The method further includes sensing a position of the beam element of the MEMS at a sensor of the MEMS, comparing the sensed position with a reference value, and generating a first digital signal in response to the comparing of the position and the reference value, where the first digital signal is at a high level while the sensed position is determined to be greater than the reference value and at a low level while the sensed position is determined to be less than the reference value. The method additionally includes generating the digital output signal based upon the first digital signal, and differentiating the first digital signal to obtain an intermediate signal, where the first feedback signal is based upon at least one of the intermediate signal and the first digital signal.
The present invention additionally relates to a method of providing a digital output signal based upon an analog input signal, where the digital output signal is electrically isolated from the analog input signal. The method includes receiving the analog input signal at a first actuator of a microelectromechanical system (MEMS), receiving a feedback signal at a second actuator of the MEMS, and generating movement of a beam element of the MEMS by way of the first and second actuators in response to the respective analog input and feedback signals. The method further includes sensing a position of the beam element of the MEMS at a sensor of the MEMS, comparing the sensed position with a reference value and a plurality of offset values at a plurality of respective comparators, and generating a first digital signal in response to the comparing of the position and the reference value and a plurality of additional digital signals in response to the comparing of the position with the plurality of offset values, respectively. The method additionally includes generating the digital output signal based upon the first digital signal, processing at a logical decision device the first digital signal and the plurality of additional digital signals to obtain a feedback bitstream signal, and generating the feedback signal from the feedback bitstream signal by way of a digital-to-analog converter (DAC).
The present invention further relates to a method of providing a digital output signal based upon an analog input signal, where the digital output signal is electrically isolated from the analog input signal. The method includes receiving the analog input signal at a first actuator of a microelectromechanical system (MEMS), receiving a feedback signal at a second actuator of the MEMS, and generating movement of a beam element of the MEMS by way of the first and second actuators in response to the respective analog input and feedback signals. The method additionally includes sensing a position of the beam element of the MEMS at a sensor of the MEMS, providing an indication of the sensed position and a reference value to a differential amplifier, providing two intermediate signals from the differential amplifier to a comparator bias circuit, generating a digital signal at the comparator bias circuit based upon the high and low output signals, and generating, based upon the digital signal, both the digital output signal and the feedback signal.
The present invention additionally relates to a method of providing a digital output signal based upon an analog input signal, where the digital output signal is electrically isolated from the analog input signal. The method includes receiving the analog input signal at a first actuator of a microelectromechanical system (MEMS), receiving a feedback signal at a second actuator of the MEMS, generating movement of a beam element of the MEMS by way of the first and second actuators in response to the respective analog input and feedback signals, and damping the movement of the beam element of the MEMS by way of a damping element. The method further includes sensing a position of the beam element of the MEMS at a sensor of the MEMS, comparing the sensed position with a reference value, and generating a digital signal in response to the comparing of the position and the reference value, where the digital signal is at a high level while the sensed position is determined to be greater than the reference value and at a low level while the sensed position is determined to be less than the reference value. The method additionally includes determining the digital output signal based upon the digital signal and generating the feedback signal based upon the digital signal.